The invention relates generally to precision loadout systems for loading coal or other particulate material into a moving train of railroad cars and, more particularly, to multiple-batch systems and methods which achieve acceptably high loading rates with smaller, and therefore less costly, facilities.
For economic reasons it is desirable to accurately load railroad cars to target weights very near the respective maximum weight capacities of the individual cars. Overloading a railroad car is undesirable because equipment damage can result. Underloading is undesirable because railroad car capacity is not efficiently utilized, and economic penalties can result.
As is for example disclosed in Campbell et al U.S. Pat. No. 4,629,392, a typical prior art loadout system for loading railroad cars with coal includes a relatively larger surge bin positioned above a relatively smaller weigh bin. Below the surge bin is a controlled charging gate for charging batches of coal from the surge bin into the weigh bin. The weigh bin is mechanically supported on load cells, such that the weight of the weigh bin and coal contained therein can be determined. Below the weigh bin is a controlled discharge gate for discharging coal from the weigh bin through a loading chute into the railroad cars as they travel past below. The surge bin is supplied by a conveyor system which runs substantially continuously while the train is traveling below past the loadout system, at a conveyor rate consistent with the average loading rate as successive railroad cars of the moving train, traveling for example at a speed of one-half mile per hour to one mile per hour (0.8 km/hr to 1.6 km/hr), are loaded.
During operation of a typical prior art coal loadout system, the charging gate is opened to charge from the surge bin into the weigh bin a batch of coal having a weight equal to the target weight of coal to be loaded into an individual railroad car, thereby making up a weighed batch of coal. Then, as the railroad car reaches its proper position below the weigh bin and loading chute, the discharge gate is opened, commencing the discharge of the weighed batch of coal from the weigh bin into the railroad car. Ideally, coal flows continuously into the railroad car, filling each railroad car evenly from front to back.
As an example, a typical prior art loadout system loads a moving train at a rate of 6,000 tons per hour (5,400 metric tons per hour). For loading railroad cars of 120 tons (109 metric tons) capacity, a weigh bin of 130 tons (118 metric tons) capacity may be provided, and a surge bin of 300 tons (272 metric tons) capacity. The capacity of the surge bin is thus sufficient to reliably and quickly charge the weigh bin with successive batches of coal as the railroad cars pass below the loadout system, without ever becoming empty. The conveyor supplying the surge bin runs continuously while a train is being loaded, and delivers coal at a corresponding rate of, for example, 6,000 tons per hour (5,400 metric tons per hour).
In order to reduce the storage capacities and therefore the size and cost of the surge bin and weigh bin, two-batch and four-batch coal loadout systems have previously been implemented. In a two-batch system, as each railroad car passes below the loadout system, the first batch of coal is discharged from the weigh bin. After the weigh bin is empty, but while coal is still flowing out of the discharge chute, the weigh bin is recharged from the surge bin to make up the second batch. The second batch is then discharged into the same railroad car. The precision to which the second batch can be made up to a particular weight determines the accuracy of loading the railcar. In a four-batch system, as each railroad car passes below the loadout system, the weigh bin is recharged three times, in addition to the initial batch. The precision to which the fourth batch can be made up to a particular weight determines the accuracy of loading the railcar.
Prior art multiple-batch coal loadout systems however suffer the disadvantage of greatly reduced loading rates. For example, a two-batch system might have the capability of loading coal at a rate of 3,000 tons per hour (2,700 metric tons per hour), while a four-batch system might have the capability of loading coal at a rate of only 2,000 tons per hour (1,800 metric tons per hour). The loading rate of a multiple-batch system is limited by a number of factors, including the time required to charge the weigh bin with successive batches without interrupting the continuous flow of coal from the discharge chute into each railroad car.
Thus a requirement is that a loadout system be capable of evenly loading the entire required amount of coal in each railroad car while the train is moving at a constant rate. This requirement particularly limits a multiple-batch loadout system which must be capable of making and weighing successive batches in a manner such that there is no gap in the flow into the railroad cars. With a double batching system, it is difficult to make the second batch up fast enough to avoid gaps in flow out of the loading chute while the railroad car is moving due to the size of the batch required and the requirement that the second batch be made up very accurately. In general, with a multiple-batch loadout system, as the size of the final batch increases, the loadout rate and therefore the train speed decreases, because it takes more time to make up a batch with precision, as is described in detail hereinbelow. But a limitation of a four-batch system is that the train speed cannot be constant, because the smaller first batch is insufficient to fill the front of a railroad car, and it may be necessary to slow or stop the train while the second batch is made up and discharged.